The invention relates to an “impregnated”-type cathode for an electron gun, that can be used in electron tubes, such as klystrons or gyrotrons, and more especially in cathode ray tubes for displaying images.
Referring to FIG. 2, a cathode of the “impregnated”-type for an electron gun comprises:                a porous cathode emissive body 1 forming that part of the cathode that undergoes thermally induced electron emission, formed from a porous matrix impregnated with an electron-emitting material;        a metal dish 2 into which the emissive body 1 is inserted;        a metal sleeve 3, preferably made of a refractory metal such as molybdenum, tantalum or tungsten, closed at one end by the dish 2; such a sleeve is also called a cathode skirt; and        inside the sleeve, a heating filament 4 that extends to a point close to the dish 2 and is suitable for heating the emissive body 1 in vacuum to a temperature of around 1000° C.        
That surface of the porous cathode emissive body which is on the opposite side from that in contact with the bottom of the dish forms the emissive surface of the cathode.
Such impregnated cathodes are used as electron sources in image display cathode ray tubes of the monitor type or in television tubes and high-definition tubes (HDTV, CDT, CRT), in microwave electron tubes of the klystron type or gyrotron type, or in other types of electron tubes for lasers, magnetron radar, amplifiers and power supplies, and ion generators and propulsion units (for satellites).
The cathode emissive body of an impregnated cathode for a display cathode ray tube has a small thickness, which limits the amount of electron-emitting material available and thus limits the lifetime of the cathode; it has been established that the lifetime characteristics of such an impregnated cathode depend on the rate of evaporation of the main component of the electron-emitting material, which is generally barium; moreover the evaporated barium recondenses on other parts of the tube that are cooler, especially the counterelectrodes of the cathode, from where it emits parasitic electrons that impair the operation of the tube; furthermore, the emitting surface of the cathode may deteriorate over the course of operation by the impact of ions, which impair the uniformity of the surface electron emission distribution.
To limit these drawbacks, the document EP 0890972 (MATSUSHITA) discloses an impregnated cathode whose cathode emissive body has a lower porosity near the emissive surface than in the core or through the depth.
For another purpose, namely to increase the resistance of the impregnated cathode to ion bombardment, the document EP 0831512 (TOSHIBA) provides, conversely, a higher porosity near the emissive surface than in the core or through the depth.
Moreover, documents JP60-017831 and JP05-114352 disclose processes for manufacturing cathode emissive bodies for an impregnated cathode, comprising a step in which, after impregnation with the electron-emitting material, the emissive surface of these bodies is abraded, especially by polishing, essentially for the purpose of cleaning this emissive surface and of removing any particles of the impregnation material from the surface; cathode emissive bodies are therefore obtained in which only the emissive surface has a low roughness, for example between 0.2 μm and 3.2 μm; no indication is given about the porosity of the surface layer immediately beneath the emissive surface, with respect to the porosity in the core of the cathode emissive body.
The document JP06-103885 (TOSHIBA) teaches that, by polishing the emissive surface so as to lower its roughness, it is possible to limit the evaporation of electron-emitting material during operation of the cathode and thus improve the operation and the lifetime.
The document U.S. Pat. No. 5,990,608 recommends a roughness of the emissive surface of less than 10 μm in order to increase the emittance of this surface (cf. FIG. 12 of that document).
To make it easier to form a film of emissive material on the emissive surface, the document EP1 063 668 teaches the polishing of this surface until a roughness of less than or equal to 3 μm, or even 1 μm, is obtained.
Finally, the document GB 1 522 387 teaches the polishing of the emissive surface in order to remove therefrom the film of barium scandate which would be formed thereon.
Some of the documents cited therefore teach that a modification of the morphology only on the emissive surface (roughness) of the cathode emissive body and/or of the layer subjacent to this surface (porosity) improves the operation of the impregnated cathode and its lifetime.